This invention relates generally to articles formed from titanium alloys, and more specifically to articles having varying microstructures in pre-selected regions and methods therefore.
Titanium fan and compressor blade dovetails are susceptible to fretting and edge of contact (EOC) fatigue failures at the dovetail/disk slot contact location due to high contact stresses. Exemplary compressor blade dovetail fatigue failures are due to a) loss of wear coating on the dovetail surface, b) lever arm failures, c) vane bushing failures, and d) stick/slip condition with the mating titanium spool.
One approach used to address the problem of premature fatigue failure is replacement of a Ti6Al4V alloy with Ti4Al4Mo2Sn (Ti442) alloy. This high strength alloy considerably reduces dovetail failures. However, recurrence of dovetail failures pose a high cost to business and further reductions in fatigue failure are sought.
Generally, fan and compressor titanium-based blades comprise equiaxed alpha+beta titanium alloys. This microstructure provides a good balance of mechanical properties for the combined airfoil/dovetail structure. It is known in the titanium industry that titanium articles having bimodal (alpha+beta) or martensitic microstructures have superior high cycle fatigue (HCF) properties compared to mill annealed titanium articles. Articles having martensitic or bimodal microstructures are slightly harder and stronger than coarse or slow cooled microstructures. An increase in the hardness and yield strength of the titanium alloys increases the resistance to crack initiation by fatigue. Thus, any improvement in the strength of the titanium alloy increases fatigue resistance including resistance to environmentally- or contact-driven fatigue.
Martensitic and bimodal structures may be obtained through high temperature heat treat followed by water quench. However, the necessary heat treat/quench cycle cannot be applied to a substantially net-shaped airfoil due to dimensional distortion during heat treat and quench. Thus, the heat treat/quench process is not feasible on a complete blade.
Accordingly, it would be desirable to provide the dovetail region of a compressor blade that capitalizes on the high strength and fatigue resistance of bimodal and/or martensitic structure while preserving the airfoil dimension and mechanical properties.